Dual-chamber pacemakers and implantable cardioverter defibrillators require manual programming of numerous programmable parameters including but not limited to: choice of pacing mode, atrioventricular delay, atrioventricular hysteresis (AV hysteresis), and other parameters such as atrial sensitivity, ventricular sensitivity, post-ventricular atrial refractory period, post-ventricular atrial blanking period, ventricular refractory period, ventricular output, atrial output, upper rate limit, base rate, sleep rate, sensor slope, sensor threshold, and so forth. The programming of these parameters can be inaccurate and time consuming, and requires skilled medical expertise to accomplish.
For example, the choice of the pacing mode is a therapeutic decision made by the medical practitioner at the time of device implant. However, the optimal pacing mode may and does change over time as the patient's clinical condition or disease state changes. For example, a patient requiring dual chamber stimulation may in fact have intermittent atrioventricular conduction. At times when atrioventricular conduction is intact, single chamber atrial stimulation, or AAI mode, has been found to be therapeutically superior to dual chamber stimulation. Generally, such a patient would be paced in a dual chamber mode, for example DDD mode, but when an intrinsic R-wave is detected, ventricular stimulation is inhibited to allow natural heart conduction to occur. Stimulation devices capable of such functional mode switching are readily available.
A problem arises, however, in that the atrioventricular interval, which is the interval that must expire without R-wave detection following an atrial stimulation pulse before a ventricular stimulation pulse is delivered, is typically programmed to a very short value during dual chamber stimulation. A relatively short atrioventricular interval, also referred to as the AV interval has been found to give hemodynamic benefit during dual chamber stimulation in some special circumstances like hypertropic cardiomyopathy, and when AV block is present but may be deleterious in the setting of normal conduction and a normal ventricular activation sequence. A short AV interval, however, is likely to be shorter than the natural atrial-ventricular conduction time (referred to as AV conduction time) of the heart. A short AV interval will preclude the detection of the intrinsic R-waves when AV conduction is intact because ventricular stimulation will occur before natural AV conduction has had time to occur. This situation may be deleterious in the setting of normal AV conduction and a normal ventricular activation sequence. The stimulation device usurps control over the natural conduction of the heart.
One disadvantage of this pacemaker competition with natural heart conduction is that natural AV conduction, when intact, has been found to be more beneficial to the patient than dual-chamber stimulation. Another disadvantage is that predominate ventricular stimulation in a patient with intact AV conduction unnecessarily wastes pacemaker battery life.
These problems have been addressed by adding positive hysteresis to the AV interval. AV hysteresis is an additional time period added to the AV interval during ventricular sensing. In essence, the interval that must expire before a ventricular stimulation pulse is delivered is extended by the AV hysteresis, allowing more time to sense for naturally conducted R-waves. Once ventricular stimulation is initiated, the ventricular stimulation pulses are then delivered at the programmed AV interval for a variable duration based on time or number of cycles.
The AV hysteresis is typically a programmable value that can be enabled or disabled. If enabled, it is commonly programmed to a setting between approximately 10 and 120 msec. Programming of the AV hysteresis, however, has been confusing to medical practitioners in that, first, the resulting stimulation rate is different than the sensing rate. Second, to determine an appropriate AV hysteresis, the AV interval must be temporarily programmed to a very long interval, then the AR interval (defined as the time interval between an atrial stimulation pulse and the subsequently sensed R-wave) must be measured. The minimum AV hysteresis is preferably the difference between the programmed AV interval and the measured AR interval. Since this measurement can be a time-consuming task, in practice, an arbitrary setting is often chosen. An arbitrary setting, however, may cause problems in that an insufficiently long hysteresis interval may result in fusion beats. Too short of an AV hysteresis setting would be ineffective because it does not allow a greater degree of R-wave detection.
Furthermore, a problem still exists, in that once ventricular stimulation is initiated at the programmed AV interval, it will continue to predominate over the natural heart rhythm. Attempts in overcoming this problem generally include temporarily extending the programmed AV interval by the AV hysteresis interval periodically during ventricular stimulation to allow for detection of an intrinsic R-wave in case AV conduction has returned. Reference is made to U.S. Pat. No. 5,814,077 to Sholder et al., U.S. Pat. No. 5,417,714 to Levine et al., and U.S. Pat. No. 5,318,594 to Limousin et al.
While these methods allow for periodic detection of restored AV conduction, the effect of temporal changes in the AV conduction time has not been fully addressed. Conventionally, the AV hysteresis is set to a fixed value that is either added to the AV interval as an additional time-out interval or not. The initial programmed value for the AV hysteresis may become inappropriate if changes in the AV conduction time occur. The AV conduction time may vary over time as a result of changes associated with the disease state of the patient, response to alterations in medical therapy, and even natural fluctuations occurring over a 24-hour period.
Thus automatically measuring the AV conduction time and monitoring variations in the AV conduction time over time would be desirable. Furthermore, automatic adjustment of the AV hysteresis based on the measured AV conduction time would be desirable to prevent pacemaker competition with natural heart conduction and thereby preserve battery life, as well as improve the performance of functional mode-switching stimulation devices in providing optimal dual-chamber stimulation therapy. As used herein, “functional mode-switching” refers, for example, to a DDD mode that behaves like one of the other modalities such as AAI, when conduction is present.
Since the AV conduction time can be different following an atrial stimulation pulse than following an intrinsic P-wave, determination of the AV conduction time following both events would be desirable so that a unique positive hysteresis interval could be determined and applied during atrial sensing (following atrial P-waves) as well as during atrial stimulation. In addition, storing the AV conduction time measurements over time so that they are available for future display would provide a valuable a diagnostic tool for the clinician in monitoring the progression of conduction disease or responses to medical therapy.